Many are saying that MSN Search is launching tomorrow. John Battelle has some interesting tidbits on it.
John Markoff writes that "Microsoft will stress the size and completeness of its service" as it's advantage over Google and Yahoo.
As I said in "It's not how big it is, it's what you do with it", improvements to search are all about relevance. The size of your index only matters if you're missing documents that should be at the top of the search results in a substantial number of cases. Otherwise, you should really be focused on getting the documents already in your index surfaced at the right times.
I'd hoped there would be more here given Microsoft Research's work in question answering, personalization, and natural language processing. We'll see what they end up releasing, but I hope this isn't just an "ours is bigger" play.
Update: Danny Sullivan posts an interesting timeline that shows MSN Search's development over the last several years.